The present invention relates to a proportional control system having reset for substantially reducing system droop and, more particularly, to a system wherein any reset windup which the system experiences is limited to a nominal value.
Original condition control systems for controlling such conditions as temperature, humidity, motor position and motor speed were primarily manually controlled systems which required an operator to be in continuous attendance to supervise the condition being controlled and make any necessary adjustments for ensuring that the condition was being controlled at the proper level. Because such systems required a human operator to continually monitor and adjust the control system, the number of control systems which could be supervised by a single human operator had to be kept within reasonable bounds. Moreover, any momentary inattention of the human operator could result in processes being ruined, motors being driven to the wrong position resulting in irregularities of the work pieces being processed by the motor control systems, and the like. In order to avoid such problems caused by inadvertence or mistake of the human operator and to relieve the human operator of many of the supervisory and control duties attendant with such control systems, the prior art developed automatic control systems.
Such automatic control, systems typically involved a manual setpoint control, which the human operator could adjust once or periodically according to a program sequence and a condition sensing mechanism for providing a feedback signal based upon the actual condition being sensed. The system compared the desired or set point level for the condition against the actual level of the condition to reduce any deviations therebetween by proper adjustment of the control system. Thus, such systems did not require constant supervision by the human operator.
Such control systems were typically proportional control systems which controlled such loads as motors and heaters in proportion to the error signal between the desired or set point level for the condition and the actual level of the condition. But the proportional control system requires an error signal between the set point level and the actual level of the condition in order to control the load for maintaining the condition at the desired level. For example, if the load is an electric heater in a process control oven, a drop in the temperature outside of the oven will mean that more current must be supplied to the heater in order to maintain the temperature within the oven at the desired level. More current can only be supplied to the heater if there is an increased error signal between the sensed condition and the desired condition. If the temperature outside of the oven drops still further, the error signal must increase in order to supply still more current to the electric heater. Thus, it can be seen that how hard the load must work to maintain the desired condition determines the amount of error signal. In order for there to be an error signal, there must be a deviation between the actual condition and the set point condition. This error signal is called "droop."
As can be seen, droop is undesired because it results from the necessary requirement that in order to energize the load at a given level to meet external conditions, there must be a deviation between the actual condition and the desired condition. Such a deviation means that the actual condition cannot equal the set point condition. In order to eliminate this droop, prior art systems incorporated reset into proportional control systems. Reset requires that, as soon as an error signal develops, there is a gradual shift of the proportional band to bring the controlled condition back to the desired level. Thus, the error signal is gradually reduced until it is substantially eliminated and the actual condition is at set point.
Unfortunately, if the load is incapable of controlling the load at a level which is required by the control system, the reset portion of the control system will continuously adjust the proportional band to a limit which is defined by the system parameters. This phenomenon is known as "reset windup." If the condition which has caused the reset windup is suddenly corrected after the limit is reached, the load then is suddenly energized fully in one direction or another which can cause as a minimum a sharp and substantial deviation in the condition being controlled by the load and, possibly, damage to the load being controlled by the proportional control system. The present invention provides a mechanism for limiting reset windup.